In the manufacturing and finishing processes of paper, paperboard and other corresponding web-like materials, the significance of methods and systems monitoring the web as such and/or the process in real time increases continuously when the aim is to achieve as high a production capacity as possible and a high and equal quality of the product.
The use of optical methods has been found to be very efficient in implementing real-time monitoring of a rapidly moving fibre web and its path. Advantages of optical methods include, for example, the possibility to perform measurements of the object in a contactless manner and to perform measurements with a rapid time response. Several examples of applying optical methods in the manufacturing and finishing processes of web-like materials are known from prior art.
In so-called imaging methods, i.e. methods which produce a visual image of an object and which are known from prior art, the object is typically recorded with matrix or line scan cameras, which are based on CCD technology (Charged Coupled Device) and thus operate on a substantially visible wavelength range. Imaging measurements may be performed as local measurements, but nowadays also as measurements monitoring the fibre web across its entire production width. Prior art is described, for example, in the conference publication “Paper Machine Applications with Fullsheet Imaging Measurement” (Chen Shih-Chin et al, Control Systems '98: Information tools to match the evolving operator role, pages 330 to 337, Porvoo, Finland, Sep. 1 to 3, 1998). In imaging measurements, the user aims at recognizing visually detectable abnormal phenomena and their causes in the object from the image either by him/herself or with the help of computer image processing.
In addition to the imaging diagnostics, which operates on the visible range, prior art also recognizes the use of thermal cameras operating in the infrared range for monitoring the fibre web and its path.
Prior art further recognizes optical methods based on spectral separation as well, wherein the characteristics of the object, e.g. paper web, are determined quantitatively by measuring and analysing certain wavelength bands (spectral bands) included by the light emitted, transmitted or reflected by the object, or a more continuous wavelength distribution of the light (continuous spectrum). It is particularly typical for spectrum separating methods that they utilize the spectral characteristics of the light measured from the object, or, more generally of the electromagnetic radiation, to determine a certain quality variable. In methods based purely on imaging, the detection of light typically occurs without actual spectrum separation by using only one wavelength band, which wavelength band is often determined according to the wavelength sensitivity of the detector.
Because the abovementioned methods, which are based on spectrum separation, can be considered to represent the closest prior art from the point of view of the current invention, they will be discussed in more detail in the following.
U.S. Pat. No. 4,733,078 presents equipment for measuring the moisture level of a paper web, wherein radiation from the infrared range is directed from a light source through the paper web in such a manner that the radiation, which has penetrated the paper web, is measured successively in time on several spectral bands to determine the moisture level of the paper web. An obvious problem of this solution is the difference in time of the measurements taken on different spectral bands, in which case when using data measured on different spectral bands at different times for determining the moisture level of the web, the measurement result obtained in this manner does not describe a certain point of the web exactly, but the measurement result is naturally affected by the movement of the web during the measurement.
U.S. Pat. No. 3,641,349 discloses equipment, in which radiation of the infrared range, which has penetrated a paper web, is divided into several optical branches, each optical branch being directed through a separate wavelength filter to a separate detector. By modulating the radiation directed through the paper web in a suitable manner, it is possible to perform the measurement simultaneously on all the examined spectral bands, i.e. in a temporally parallel manner, in which case the moisture value of the web, obtained as a result of the measurement, better represents a certain local range of the web. One limitation of the method is that the number of spectral bands being examined is, in practice, limited, because the complexity of the measurement equipment increases significantly with an increase in the number of spectral bands. Because of the number of detectors the costs of measurement equipment can also be high.
The applicant's previous patent application WO 99/14579, which has become public, further discloses measurement equipment, wherein infrared radiation is directed from the light source through a moving paper web, and the radiation, which has penetrated the paper web, is further led to a spectrograph for spectrum separation. By means of this arrangement it is possible to measure, instead of a few separate spectral bands, a more continuous spectrum of the radiation with the help of a matrix detector installed on the output level of the spectrograph. Thus, in the measurement, all the examined so-called wavelength channels are measured at the same time, i.e. temporally in parallel with each other. Thus, as the measurement result the momentary continuous spectrum of the measuring point can be determined in a certain wavelength range, in which case the accuracy of determining the characteristics of an object, such as the moisture level of the web or other examined quality variable, on the basis of the aforementioned spectral data, is better than when determining the quality variable on the basis of only a few spectral bands, which are measured, for example, by using separate and parallel optical filters.
U.S. Pat. No. 5,019,710 discloses equipment, by means of which the measurement based on spectral separation can be performed across the entire cross-directional width of the fiber web without needing to traverse the optical measurement device, i.e. to move it mechanically in cross-direction of the web. The solution presented in the U.S. Pat. No. 5,019,710 is based on the use of optical fibres in such a manner that the measuring point is traversed for different points in the cross-direction of the web by multiplexing the optical device, which performs the actual measurement and comprises a light source and a detector part, to optical fibre pairs arranged at different points of the web, in which optical fibre pairs one of the optical fibres placed on different sides of the web conducts light from the light source to the web, and correspondingly the other conducts the light from the web to the detector part.
Furthermore, publication WO 99/02941 presents the measuring of the thickness of a paper web in such a manner that a measuring beam is led to the paper web with optical fibres, and the radiation, which has penetrated the web, is further conducted to an optical measurement device. In this solution, the optical fibres are arranged side by side in the cross-direction of the web substantially across the whole width in the transverse direction of the web. According to one embodiment presented in the publication, measurement of the continuous spectrum in the cross-direction of the web in all measuring points is performed simultaneously by means of parallel miniature spectrometers.
In a summary, it can be stated that by using the arrangements of prior art in order to determine the characteristics of the web, it is possible to implement different kinds of spectrum separated optical measurements in such a manner that the measurements cover substantially the entire cross-directional width of the web and either individual spectral bands or a continuous wider spectral range is measured at each measuring point. Temporally, the measurement can be implemented in such a manner that the measurement of the cross-directional profile of spectral data is implemented in the transverse direction of the web at all measuring points substantially at the same time (without traversing), and at each measuring point all the wavelengths being monitored are registered substantially at the same time (in a temporally parallel manner).
In the above-described spectrum separated measurements according to prior art, the time elapsed in the measuring, during which time the web being examined thus continuously travels past the monitoring point, is, according to the opinion of the applicant, discussed only in the following senses in the development of measurement methods. Firstly, there is an aim to develop the measurement methods in such a manner that the measurement information is registered simultaneously across the entire width in the transverse direction of the web without mechanical traversing or the like. The aim is thus for the measurement to efficiently cover the entire area of the web. Secondly, there is an aim to develop the measurement methods in such a manner that the individual spectral bands or the continuous spectrum being examined in each cross-direction measuring point of the web are to be measured temporally at the same time, in which case an interesting characteristic of the web that is to be determined by using them, such as moisture, represents the point of the web more accurately without the movement of the web affecting the measurement result.
Basic Principle and Most Important Advantages of the Invention
The primary aim of the present invention is to provide a novel method for controlling quality and condition on the basis of spectrum separating measurement, to be used in processes for manufacturing and/or finishing a continuous fibre web.
The aim of the invention is to utilize, significantly more efficiently than in prior art, the possibility to perform optical spectrum separated measurements on the web or its path, with a good temporal and spatial resolution. The aim of the invention is thus further to make it possible to collect significantly more diverse and detailed data from an object being measured than with prior art methods, and further to automatically analyse the measurement results significantly more efficiently with a computer or the like.
It is a particular aim of the invention to make it possible to observe more accurately than in prior art, and reliably identify fast phenomena, which appear only momentarily in the so called machine direction in the process. The invention is also applicable, however, for analysing more continuous phenomena. A very significant advantage of the invention is that the cause of failures in the process can now be identified by means of the invention in an easier and more reliable way than in prior art. Thus, the invention makes it possible to implement a highly automated control of quality and condition.
The essential basic idea of the invention is that temporally and spatially resolved spectral data is collected with the optical spectrum separating measurement device from a moving object, i.e. a web or an element involved in handling it, in a manner synchronized with the movement of the object. On the basis of this spectral data, a two-dimensional “quality variable chart” is formed, which chart is further substantially continuous in the direction of movement and describes at least one quality variable.
In this context, the two-dimensional quality variable chart refers to a method of storing data, in which the data is stored in a coordinate system, its first axis describing a point in the cross-direction of the object (such as, for example, the web), and the second axis of the coordinate system describing a point in the direction of movement of the object. At each point, which is determined with the help of the two coordinates data on the value of the quality variable is stored.
The quality variable can be, for example, the moisture level of the web or the amount of coating of the web, which quality variables are determined from the web on the basis of, for example, transmission and spectrum separated reflection measurements, which are carried out on the infrared range. Thus the quality variable chart reveals, for example, changes in the moisture level of the web at different points of the web in machine direction and cross-direction.
The quality variable being examined can also be another characteristic of the web, the coating of the web or an element connected to the handling of the web, which is determined, for example, by means of spectrum separated measurements performed on the visible range. The quality variable can thus be, for example, the colour, opacity, brightness, gloss, or smoothness of the object.
According to the invention, to determine the characteristics of the object and/or in order to detect defects in the object, local deviations and/or discontinuities, especially in the direction of motion of the object, of at least one quality variable are further recognized in the continuous quality variable chart, and the cause of such deviances and/or discontinuities is recognized on the basis of the periodicity of the phenomena in the direction of the movement.
Thus, synchronizing the two-dimensional quality variable chart with the movement of the object will make it possible to detect, in the quality variable chart, recurrent and transient phenomena in the machine direction, in an efficient and, if necessary, automatic way, and furthermore, the causes of these recurrent phenomena can be identified on the basis of the periodicity of the phenomena.
If the object to be measured is, for example, a moving fibre web, the cause of a defect recurring at regular intervals in the web can be identified to be a roll rotating at a known peripheral speed in relation to the web, wherein a damaged or soiled point on the surface of the roll causes a defect in the passing web, recurring at intervals corresponding to the peripheral length of the roll.
In an advantageous embodiment of the invention, the quality variable chart formed of the object or the spectral data used in forming it is averaged in the machine direction across a cycle length specific to the object to be monitored, in order to detect periodical phenomena of lower intensity. The cycle length to be used in the averaging can be selected to correspond, for example, to the peripheral length of a specific roll. Averaging efficiently removes random noise present in the measurement result.
By using the method according to the invention, it is possible to search, in the quality variable chart, for periodical phenomena caused by the object to be measured, or by a component preceding it in the process. Thanks to the collecting of spectral data at good sensitivity, and averaging of the results, it is also possible to detect periodical phenomena in the fibre web, which are caused by (preceding) components located farther away from the point of measurement. As a result, it is possible to monitor more steps (a longer web length) in the process by using only a single measuring point.
The method according to the invention can be applied for monitoring the moving web itself, or for monitoring the condition of rotating/moving means which are involved in the processing of the web and are in contact with it, such as rolls and various textures (wires, felts). The invention is also applicable for monitoring the properties of reels to be formed of the web.
A malfunction of the means involved in the processing of the fibre web can be detected either by direct measurement of the means or on the basis of a marking caused in the passing web by the means.
The measurement according to the invention can be carried out in relation to the direction of movement of the object being examined across the entire width in the transverse direction of the object or only partly across the width. When measuring across the entire width of the object, for example a fibre web, the measurement is carried out advantageously by using such a measurement arrangement, where the measurement of the cross-direction profile of the spectral data is carried out substantially at the same time across the entire width of the web. Thus the phenomena occurring in the cross-direction in relation to the direction of movement of the object and in the machine direction according to the movement can be reliably distinguished from each other.
The following more detailed description of the invention with examples will more clearly illustrate, for anyone skilled in the art, preferred embodiments of the invention as well as advantages to be achieved with the invention in relation to background art.